1. Field of the Invention
This invention relates to a catalytic convertor for treating exhaust gas.
2. Description of the Prior Art
As a catalyst for treating exhaust gas, there has been wide known a ternary catalyst which oxidizes CO and HC and reduces NOx. The ternary catalyst comprises, for instance, .gamma.-alumina bearing thereon Pt and Rh and is most effective when the air-fuel ratio is equal to the stoichiometric air-fuel ratio (i.e., 14.7).
On the other hand, from the viewpoint of fuel economy, it is preferred that the air-fuel ratio be higher than the stoichiometric air-fuel ratio. However when the air-fuel ratio is lean, oxygen concentration in exhaust gas becomes too high and the ternary catalyst cannot reduce NOx though it can oxidize CO and HC.
Recently, a zeolite catalyst bearing thereon transition metal coated by ion exchange is under intensive investigation. Such a zeolite catalyst can decompose NOx into N.sub.2 and O.sub.2 by catalytic cracking directly or by existence of a reducing agent (e.g., CO, HC or the like) even in a lean atmosphere where oxygen concentration in exhaust gas is high.
For example, it is proposed in Japanese Unexamined Patent Publication No. 1(1989)-310742 to improve NOx treating performance in a wide temperature range from a low temperature to a high temperature by providing a catalytic layer of zeolite with Cu by ion exchange and with precious metal.
However such a precious-metal-bearing Cu-ion exchange zeolite catalyst does not have an enhanced activity though having a widened active temperature range.